“Night Soil”, which is untreated, uncomposted human waste (aka poo) was commonly used as a fertiliser in the United Kingdom until around 60 years ago. It’s still used in China and some other countries, but comes with risks as faeces can be a vector of human disease transmission.
Today, science has shown us that composting human manure will render it safe in addition to compost being generally better for the soil than manure (better structure, less nutrient run-off etc).
Using a compost toilet is not taking us backwards hundreds of years, because we apply modern scientific rigour and know that composting is an essential and desirable aspect to dealing with human ‘waste’ and turning it into a safe and valuable resource.
Creating and using ‘humanure’ compost is an essential aspect of a natural, circular economy in which we can start to balance outputs and inputs.